U.S. patent application number 12/592641 was filed with the patent office on 2010-07-08 for method and apparatus for using multiple relative reflectance measurements to determine properties of a sample using vacuum ultra violet wavelengths.
This patent application is currently assigned to MetroSol, Inc.. Invention is credited to Phillip Walsh.
Application Number | 20100171959 12/592641 |
Document ID | / |
Family ID | 41012952 |
Filed Date | 2010-07-08 |
United States Patent
Application |
20100171959 |
Kind Code |
A1 |
Walsh; Phillip |
July 8, 2010 |
Method and apparatus for using multiple relative reflectance
measurements to determine properties of a sample using vacuum ultra
violet wavelengths
Abstract
A method and apparatus is disclosed for measuring properties of
an unknown sample. A reflectometer and one or more reference pieces
is provided. A set of data is collected from the unknown sample and
a combination of the reference pieces. A combination of the sample
and reference piece data independent of incident intensity is used
to determine a property of the unknown sample without calibrating
incident reflectometer intensity. The method and apparatus
disclosed can measure properties of thin films or scattering
structures on semiconductor work pieces. In one embodiment the
reflectometer utilizes vacuum ultraviolet (VUV) wavelength
reflectometry. Multiple relative reflectance measurements are used
to overcome effects of the inevitable contamination buildup that
occurs when using optical systems in the VUV region. While
advantageous for VUV wavelengths, the method described herein is
generally applicable to any wavelength range, and is advantageous
in situations where stable reference samples are not available.
Inventors: |
Walsh; Phillip; (Austin,
TX) |
Correspondence
Address: |
D. Kligler I.P. Services LTD
P.O. Box 25
Zippori
17910
IL
|
Assignee: |
MetroSol, Inc.
|
Family ID: |
41012952 |
Appl. No.: |
12/592641 |
Filed: |
November 30, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12072878 |
Feb 28, 2008 |
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12592641 |
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Current U.S.
Class: |
356/448 |
Current CPC
Class: |
G01N 21/33 20130101 |
Class at
Publication: |
356/448 |
International
Class: |
G01N 21/55 20060101
G01N021/55 |
Claims
1. A method of measuring properties of an unknown sample,
comprising: providing a reflectometer and at least one reference
sample, wherein the at least one reference sample is unstable under
conditions in which the reflectometer is operated; collecting a set
of data from the unknown sample and at least one reference sample;
and utilizing a combination of the unknown sample and reference
sample data that is independent of incident intensity to determine
a property of the unknown sample, without calibrating incident
reflectometer intensity.
2. The method of claim 1, wherein the data obtained from the
unknown sample and the at least one reference sample includes
intensity data.
3. The method of claim 2, wherein reflectance ratios are obtained
from the intensity data.
4. The method of claim 3, wherein one or more properties of the
unknown sample are obtained by analyzing reflectance ratios using
thin film models and a regression analysis to determine one or more
properties of one or more of the unknown sample and the at least
one reference sample.
5. The method of claim 2, wherein the collecting a set of data from
the unknown sample and at least one reference sample comprises
collecting a set of data from the unknown sample and a plurality of
reference samples.
6. The method of claim 5, wherein plurality of reference samples
comprises at least a first and second reference sample wherein the
first reference sample comprises a relatively thick SiO.sub.2/Si
film structure and the second reference sample comprises a native
SiO.sub.2/Si film structure.
7. The method of claim 5, wherein the reflectance ratios comprise
at least one ratio with data from the relatively thick SiO.sub.2/Si
film structure in the numerator and data from the unknown sample in
the denominator and another ratio with data from the relatively
thick SiO.sub.2/Si film structure in the numerator and data from
the native SiO.sub.2/Si film structure in the denominator.
8. The method of claim 5, wherein a contaminant layer is included
in a model for the reference samples.
9. The method of claim 5, wherein the unknown sample is an
ultra-thin silicon oxynitride or hafnium-silicide film.
10. The method of claim 1, wherein the reflectometer is operated in
at least vacuum ultraviolet (VUV) wavelengths and the at least one
reference sample is unstable under VUV conditions.
11. The method of claim 10, wherein the data obtained from the
unknown sample and the at least one reference sample includes
intensity data.
12. The method of claim 11, wherein reflectance ratios are obtained
from the intensity data.
13. The method of claim 12, wherein one or more properties of the
unknown sample are obtained by analyzing reflectance ratios using
thin film models and a regression analysis to determine one or more
properties of one or more of the unknown sample and the at least
one reference sample.
14. The method of claim 11, wherein the collecting a set of data
from the unknown sample and at least one reference sample comprises
collecting a set of data from the unknown sample and a plurality of
reference samples.
15. A system for measuring properties of an unknown sample,
comprising: at least one reference sample; a reflectometer
configured for collecting a set of data from the unknown sample and
the at least one reference sample wherein the at least one
reference sample is unstable under conditions in which the
reflectometer is operated; and a computer operating a software
routine configured to utilize a combination of the unknown sample
and reference sample data that is independent of incident intensity
to determine a property of the unknown sample, without calibrating
incident reflectometer intensity.
16. The system of claim 15, wherein the at least one reference
sample is a reference piece integrated with a sample holding
system.
17. The system of claim 16, further comprising of a plurality of
the reference pieces.
18. The system of claim 15, wherein the data obtained from the
unknown sample and the at least one reference sample includes
intensity data and wherein the software routine is configured to
obtain reflectance ratios from the intensity data.
19. The system of claim 15, wherein the at least one reference
sample comprises a plurality of reference samples.
20. The system of claim 19, wherein the plurality of reference
samples comprises at least a first and second reference sample
wherein the first reference sample comprises a relatively thick
SiO.sub.2/Si film structure and the second reference sample
comprises a native SiO.sub.2/Si film structure.
21. The system of claim 15, wherein the reflectometer is configured
to operate in at least vacuum ultraviolet (VUV) wavelengths and the
at least one reference sample is unstable under VUV conditions.
22. The system of claim 21, wherein the at least one reference
sample is a reference piece integrated with a sample holding
system.
23. The system of claim 22, further comprising of a plurality of
the reference pieces.
24. The system of claim 21, wherein the data obtained from the
unknown sample and the at least one reference sample includes
intensity data and wherein the software routine is configured to
obtain reflectance ratios from the intensity data.
25. The system of claim 21, wherein the at least one reference
sample comprises a plurality of reference samples.
26. A system for measuring properties of an unknown sample,
comprising: at least one reference sample; a reflectometer,
configured for collecting a set of data from the unknown sample and
the at least one reference sample wherein the at least one
reference sample is unstable under conditions in which the
reflectometer is operated; and a computer operating a software
routine that selectably operates in at least one of a plurality of
measurement modes, the plurality of measurement modes including at
least a first measurement mode and a second measurement mode,
wherein, the first measurement mode is configured to utilize a
combination of the unknown sample and reference sample data that is
independent of incident intensity to determine a property of the
unknown sample, without calibrating incident reflectometer
intensity, and the second measurement mode is configured to utilize
the reference sample data in a manner that is independent of
incident intensity to determine one or more properties of one or
more reference pieces, thereby determining the incident intensity
of the reflectometer, after which reflectance of unknown samples
may be determined.
27. The system of claim 26, wherein the at least one reference
sample is a reference piece integrated with a sample holding
system.
28. The system of claim 27, further comprising of a plurality of
the reference pieces.
29. The system of claim 26, wherein in the first measurement mode
the data obtained from the unknown sample and the at least one
reference sample includes intensity data and wherein the software
routine is configured to obtain reflectance ratios from the
intensity data.
30. The system of claim 26, wherein in at least the first
measurement mode the at least one reference sample comprises a
plurality of reference samples.
31. The system of claim 30, wherein in the first measurement mode
the plurality of reference samples comprises at least a first and
second reference sample wherein the first reference sample
comprises a relatively thick SiO.sub.2/Si film structure and the
second reference sample comprises a native SiO.sub.2/Si film
structure.
32. A method of measuring properties of an unknown sample,
comprising: providing a reflectometer and at least one reference
sample, wherein the at least one reference sample is unstable under
conditions in which the reflectometer is operated; collecting a set
of data from the unknown sample and at least one reference sample;
and selectably operating the system in at least one of a plurality
of measurement modes, the plurality of measurement modes including
at least a first measurement mode and a second measurement mode,
wherein, the first measurement mode is configured to utilize a
combination of the unknown sample and reference sample data that is
independent of incident intensity to determine a property of the
unknown sample, without calibrating incident reflectometer
intensity, and the second measurement mode is configured to utilize
the reference sample data in a manner that is independent of
incident intensity to determine one or more properties of one or
more reference pieces, thereby determining the incident intensity
of the reflectometer, after which reflectance of unknown samples
may be determined.
33. The method of claim 32, wherein in the first measurement mode
the data obtained from the unknown sample and the at least one
reference sample includes intensity data.
34. The method of claim 33, wherein in the first measurement mode
reflectance ratios are obtained from the intensity data.
35. The method of claim 34, where the reflectance ratios comprise
at least one ratio with data from the relatively thick SiO.sub.2/Si
film structure in the numerator and data from the unknown sample in
the denominator and another ratio with data from the relatively
thick SiO.sub.2/Si film structure in the numerator and data from
the native SiO.sub.2/Si film structure in the denominator.
36. The method of claim 32, wherein in the first measurement mode
one or more properties of the unknown sample are obtained by
analyzing reflectance ratios using thin film models and a
regression analysis to determine one or more properties of one or
more of the unknown sample and the at least one reference
sample.
37. The method of claim 32, wherein the collecting a set of data
from the unknown sample and at least one reference sample comprises
collecting a set of data from the unknown sample and a plurality of
reference samples.
38. The method of claim 32, wherein the at least one reference
sample comprises at least a first and second reference sample
wherein the first reference sample comprises a relatively thick
SiO.sub.2/Si film structure and the second reference sample
comprises a native SiO.sub.2/Si film structure.
39. The method of claim 32, where a contaminant layer is included
in a model for the reference samples.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 12/072,878 filed on Feb. 28, 2008.
TECHNICAL FIELD
[0002] A method and apparatus for using reflectometry for measuring
properties of thin films or scattering structures on semiconductor
work-pieces is disclosed. The techniques described herein include a
method for using multiple relative reflectance measurements to
overcome effects of contamination buildup. While the methods are
particularly advantageous for vacuum ultraviolet (VUV) wavelengths,
the methods are generally applicable to any wavelength range, and
are advantageous in situations where stable reference samples are
not available.
BACKGROUND
[0003] The techniques described herein relate to the field of
optical metrology. Optical methods for control of thin film
properties in semiconductor (and other) device manufacturing
environments have become widely accepted. Particular advantages of
using optical metrology include a high measurement throughput and
the fact that optical measurements are typically
nondestructive.
[0004] The most common optical metrology techniques are
reflectometry and ellipsometry. Ellipsometry is generally regarded
as consisting of a "richer" dataset, including a measurement of two
quantities per wavelength/incident angle. On the other hand,
reflectometers are more robust due to less complex hardware
configuration, have faster measurements, and typically have a
smaller footprint. Generally speaking, if both technologies are
capable of solving a given metrology problem, the reflectometer is
a more cost effective choice for a high-volume production
environment.
[0005] Semiconductor device manufacturing is characterized by
continually decreasing feature sizes. For example, in integrated
circuit (IC) devices, the shrinking of the gate length has caused a
corresponding decrease in the gate dielectric thickness to the
order of 1 nm. Consequently, an important manufacturing issue is
control of properties of ultra-thin films such as for example
silicon oxynitrides or hafnium silicate films. Usually, control of
film thickness is of primary importance, but control of film
composition can be equally important, since both properties
influence the final IC device performance.
[0006] This shrinking of device dimensions is where vacuum
ultra-violet wavelength metrology comes in. It is well-known that a
decrease in incident wavelength enhances sensitivity of the
detected signal to minute changes in samples properties. An example
is reflectance of .about.1-2 nm silicon dioxide films on silicon
substrates. FIGS. 1A and 1B compare simulated reflectances of 10
.ANG. SiO.sub.2/Si film (plot 101), 11 .ANG. SiO.sub.2/Si film
(plot 102), and 12 .ANG. SiO.sub.2/Si film (plot 103). Changes in
film thickness are only detectable in the deep-ultra violet (DUV)
and VUV regions, are more resolved the shorter the wavelength, and
are undetectable in the visible wavelength regions. FIG. 1A shows a
reflectance range of 30% to 80% and a wavelength range of 120 nm to
1000 nm, while FIG. 1B is an expanded version of a portion of FIG.
1A, with a reflectance range of 45% to 70% and a wavelength range
of 120 nm to 220 nm. The differences between the reflectances of
plots 101, 102 and 103 are more apparent in FIG. 1B.
[0007] Somewhat less known in the art is the ability to distinguish
the effects of multiple parameters on the detected spectrum as the
incident wavelength decreases below DUV regions. The ability to
determine changes in film thickness and composition independently
is enhanced in the VUV region, where many films exhibit very rich
absorption spectra. Thus, using only DUV wavelengths, it may be
possible to distinguish thickness or composition changes in an
ultra-thin film, but not simultaneously. To do this with a
reflectometer, one must move to VUV wavelengths, as illustrated in
"Optical characterization of hafnium-based high-k dielectric films
using vacuum ultraviolet reflectometry" (C. Rivas, XV International
Conference on Vacuum Ultraviolet Radiation Physics, published 2007)
for the case of Hf.sub.xSi.sub.1-xO.sub.2, or in FIGS. 2A-C for
silicon oxynitride (SiON). FIGS. 2A, 2B, and 2C compare
reflectances for three SiON film cases: 30 .ANG. thick, 15% nitride
component (plot 201), 31 .ANG. thick 15% nitride component (plot
202), and 30 .ANG. thick, 17% nitride component (plot 203). FIG. 2A
shows a reflectance range of 10% to 80%, and a wavelength range of
120 nm to 1000 nm. FIG. 2B shows an expanded version of a portion
of FIG. 2A, with a reflectance range of 15% to 55%, and a
wavelength range of 120 nm to 160 nm. FIG. 2C shows a second
expanded version of a portion of FIG. 2A, with a reflectance range
of 60% to 70%, and a wavelength of 180 nm to 300 nm. FIG. 2B shows
that VUV reflectance can be used to distinguish all three films.
FIG. 2C illustrates how DUV reflectance can distinguish the first
film from the other two, but cannot distinguish the change of 1
.ANG. thickness from a change of 2% nitride component. In addition,
the variety and richness of absorption structure in the VUV for
many dielectric materials means that reflectance data often
contains as much as or even more information than ellipsometric
data, even when the data is taken from the same wavelength region.
FIG. 2D shows the optical parameters, n and k, for the oxide and
nitride components of the oxynitride film. In FIG. 2D, n SiO2 plot
206, k SiO2 plot 207, n Si3N4 plot 208, and k Si3N4 plot 209 are
shown. The large difference in absorption properties (as indicated
in the k spectra) in the VUV regions is a key enabler for VUV
reflectometery.
[0008] Consequently, a VUV reflectometer has been disclosed in U.S.
Pat. Nos. 7,026,626, 7,067,818, 7,126,131, and 7,271,394, the
disclosures of which are expressly incorporated herein by reference
in their entirety. This reflectometer has overcome the difficulties
involved with VUV operation, and in particular incorporates an
inert gas environment, as well as a real-time reference procedure
to enhance stability.
[0009] A formidable obstacle to stable, reliable metrology at VUV
wavelengths is a buildup of contaminants on optical surfaces during
operation. This contaminant buildup is generally characteristic of
all optical systems operating in the VUV region, and has also been
observed in initial 157 nm lithographic systems, as seen in
"Contamination rates of optical surface at 157 nm in the presence
of hydrocarbon impurities", (T. M. Bloomstein, V. Liverman, M.
Rothschild, S. T. Palmacci, D. E. Hardy, and J. H. C. Sedlacek,
Optical Microlithography XV, Proceedings of the SPIE, Vol. 4691, p.
709, published 2002) and "Contamination monitoring and control on
ASML MS-VII 157 nm exposure tool", (U. Okoroanyanwu, R. Gronheid,
J. Coenen, J. Hermans, K. Ronse, Optical Microlithography XVII,
Proceedings of the SPIE, Vol. 5377, p. 1695, published 2004), as
well as space-based VUV experiments, such as "Optical
Characterization of Molecular Contaminant Films", (Photonics Tech
Briefs, January 2007). For fab production environments, the
contaminant is thought to involve a photodeposition process as VUV
light interacts with siloxanes, hydrocarbons, and other compounds
common in fab environments.
[0010] One method for calibrating a VUV reflectometer system that
takes into account contaminant buildup has been disclosed in U.S.
patent application Ser. No. 10/930,339 filed on Aug. 31, 2004, Ser.
No. 11/418,827 filed May 5, 2006 (now U.S. Pat. No. 7,282,703),
Ser. No. 11/418,846 filed May 5, 2006, and Ser. No. 11/789,686,
filed on Apr. 25, 2007, which are all expressly incorporated herein
by reference in their entirety. This method involves using a
reflectance ratio, which is independent of incident system
intensity, to measure properties of contaminant layers on the
calibration samples. The measured contaminant layer properties are
used to calculate the reflectance spectra of the calibration
samples, which enables the determination of the incident intensity
from the intensity reflected from the calibration sample. Once the
incident intensity is known, an absolute reflectance can be
measured for any subsequent sample.
SUMMARY
[0011] The techniques disclosed herein provide an alternate method
(distinct from the above mentioned U.S. patent application Ser.
Nos. 10/930,339, 11/418,827, 11/418,846, and 11/789,686) of
measurement using reflectometry that bypasses system calibration
and utilizes multiple reflectance ratios, independent of system
intensity, to simultaneously measure the properties of an unknown
sample and the contaminant buildup on reference surfaces. The
method can provide better long-term measurement stability for some
ultra-thin film measurements. In one embodiment the reflectometer
utilizes vacuum ultraviolet (VUV) wavelength reflectometry.
[0012] In one embodiment a method of measuring properties of an
unknown sample is provided. The method may comprise providing a
reflectometer and at least one reference sample, wherein the at
least one reference sample is unstable under conditions in which
the reflectometer is operated, collecting a set of data from the
unknown sample and at least one reference sample, and utilizing a
combination of the unknown sample and reference sample data that is
independent of incident intensity to determine a property of the
unknown sample, without calibrating incident reflectometer
intensity.
[0013] In another embodiment a system for measuring properties of
an unknown sample is provided. The system may comprise at least one
reference sample and a reflectometer, configured for collecting a
set of data from the unknown sample and the at least one reference
sample wherein the at least one reference sample is unstable under
conditions in which the reflectometer is operated. The system may
also comprise a computer operating a software routine configured to
utilize a combination of the unknown sample and reference sample
data that is independent of incident intensity to determine a
property of the unknown sample, without calibrating incident
reflectometer intensity.
[0014] In another embodiment a system for measuring properties of
an unknown sample, may comprise at least one reference sample and a
reflectometer configured for collecting a set of data from the
unknown sample and the at least one reference sample wherein the at
least one reference sample is unstable under conditions in which
the reflectometer is operated. The system may further comprise a
computer operating a software routine that selectably operates in
at least one of a plurality of measurement modes, the plurality of
measurement modes including at least a first measurement mode and a
second measurement mode. The first measurement mode is configured
to utilize a combination of the unknown sample and reference sample
data that is independent of incident intensity to determine a
property of the unknown sample, without calibrating incident
reflectometer intensity. The second measurement mode is configured
to utilize the reference sample data in a manner that is
independent of incident intensity to determine one or more
properties of one or more reference pieces, thereby determining the
incident intensity of the reflectometer, after which reflectance of
unknown samples may be determined.
[0015] In yet another embodiment, a method of measuring properties
of an unknown sample, may comprising providing a reflectometer and
at least one reference sample, wherein the at least one reference
sample is unstable under conditions in which the reflectometer is
operated and collecting a set of data from the unknown sample and
at least one reference sample. The method further comprises
selectably operating the system in at least one of a plurality of
measurement modes, the plurality of measurement modes including at
least a first measurement mode and a second measurement mode. The
first measurement mode is configured to utilize a combination of
the unknown sample and reference sample data that is independent of
incident intensity to determine a property of the unknown sample,
without calibrating incident reflectometer intensity. The second
measurement mode is configured to utilize the reference sample data
in a manner that is independent of incident intensity to determine
one or more properties of one or more reference pieces, thereby
determining the incident intensity of the reflectometer, after
which reflectance of unknown samples may be determined.
[0016] As described below, other features and variations can be
implemented, if desired, and a related method can be utilized, as
well.
DESCRIPTION OF THE DRAWINGS
[0017] It is noted that the appended drawings illustrate only
exemplary embodiments of the techniques disclosed herein and are,
therefore, not to be considered limiting of its scope, for the
techniques disclosed herein may admit to other equally effective
embodiments.
[0018] FIGS. 1A and 1B illustrate the effect of changing oxide
thickness on reflectance for an ultra thin SiO.sub.2 film on
silicon substrate;
[0019] FIGS. 2A-2D illustrate the effect of changing SiON thickness
and percent nitride component on reflectance, as well as optical
spectra for the oxide and nitride components of an ultra thin SiON
film on silicon substrate;
[0020] FIG. 3 illustrates a practical embodiment of the current
invention, including a movable stage with sample holder and two
reference pieces;
[0021] FIGS. 4A and 4B illustrate variation in a reflectance ratio
due to changing native oxide thickness;
[0022] FIGS. 5A and 5B illustrate variation in a reflectance ratio
due to changing .about.1000 .ANG. SiO.sub.2 thickness;
[0023] FIGS. 6A and 6B illustrate variation in a reflectance ratio
due to changing contaminant thickness on both a .about.1000 .ANG.
SiO.sub.2 on silicon sample and native oxide on silicon sample;
[0024] FIGS. 7A and 7B illustrate variation in a reflectance ratio
due to changing contaminant thickness on the native oxide
sample;
[0025] FIGS. 8A and 8B illustrate variation in a reflectance ratio
due to changing contaminant thickness on the .about.1000 .ANG.
SiO.sub.2 sample;
[0026] FIGS. 9A and 9B illustrate variation in a reflectance ratio
of .about.1000 .ANG. SiO.sub.2 on silicon and an ultra thin SiON on
silicon sample due to changing SiO.sub.2 thickness;
[0027] FIGS. 10A and 10B illustrate variation in a reflectance
ratio of a .about.1000 .ANG. SiO.sub.2 on silicon and an ultra thin
SiON on silicon sample due to changing contaminant thickness on the
.about.1000 .ANG. SiO.sub.2 sample;
[0028] FIGS. 11A and 11B illustrate variation in a reflectance
ratio of a .about.1000 .ANG. SiO.sub.2 on silicon and an ultra thin
SiON on silicon sample due to changing SiON thickness;
[0029] FIGS. 12A and 12B illustrate variation in a reflectance
ratio of a .about.1000 .ANG. SiO.sub.2 on silicon and an ultra thin
SiON on silicon sample due to changing SiON percent nitrogen
content;
[0030] FIGS. 13A and 13B illustrate a measured and fit reflectance
ratio of a .about.1000 .ANG. SiO.sub.2 on silicon and a native
oxide on silicon sample;
[0031] FIGS. 13C and 13D illustrate a measured and fit reflectance
ratio of a .about.1000 .ANG. SiO.sub.2 on silicon and an ultra thin
SiON sample; and
[0032] FIG. 14 illustrates a practical embodiment of the current
invention including a moving stage with sample holder and several
mounted reference pieces, each having distinct film structure.
DETAILED DESCRIPTION
[0033] The techniques described herein provide a method and
apparatus for reflectometry for measuring properties of thin films
or scattering structures on semiconductor work-pieces. In one
embodiment vacuum ultraviolet (VUV) wavelength (or lower)
reflectometry may be utilized.
[0034] Reflectance or ellipsometric data from ultra-thin gate
dielectrics are often modeled using an effective medium
approximation (EMA), as shown in "The accurate determination of
optical properties by ellipsometry", (D. Aspnes, Handbook of
Optical Constants of Solids Volume I, ed. D. Palik, Academic Press,
San Diego, published 1998) that combines two or more constituent
components using a single volumetric fraction parameter. Such an
approximation is strictly valid when the film dimensions are much
smaller than the incident wavelength. Additionally, most EMA
approximations make further assumptions about the geometric
arrangements of the component materials. For example, the Bruggeman
EMA model assumes that the material is a composite mixture of
distinct regions, with each region having its own well-defined set
of optical properties.
[0035] Even if this assumption is not strictly met, for ultra-thin
silicon oxynitrides or hafnium silicates, treatment with the
Bruggeman EMA model adequately describes the reflectance or
ellipsometric data. Additionally, the volume fraction correlates
well with the dominant changes in composition, such as percent
nitrogen in a silicon oxynitride film. Consequently, for the
purposes of this disclosure silicon oxynitride films will be
treated as a Bruggeman EMA mixture of SiO.sub.x and Si.sub.xN.sub.y
components, while hafnium silicate films are modeled as Bruggeman
EMA mixtures of HfO.sub.x and SiO.sub.x components. It is
understood that any suitable model could be used in place of the
EMA model, and that many film systems could be similarly treated,
not limited to silicon oxynitrides and hafnium silicates.
Additionally, the methods discussed herein are not limited to just
thin film structures, but can also include scattering structures.
In particular, the unknown sample could include 1-D or 2-D grating
structures, which could be modeled using rigorous diffraction
algorithms such as the rigorous coupled wave method.
[0036] So described, a model of a silicon oxynitride film consists
of the film thickness and EMA mixing fraction of oxide (SiO.sub.x)
and nitride (Si.sub.xN.sub.y) components. The oxide and nitride
components themselves are described by their optical properties,
index of refraction n and extinction coefficient k, as functions of
wavelength. Given the film's thickness and EMA fraction, the
reflectance can be calculated at any wavelength using standard thin
film Fresnel equations, as described in "Spectroscopic Ellipsometry
and Reflectometry--A User's Guide", (H. Tompkins and W. McGahan,
John Wiley & Sons Press, New York, published 1999). A metrology
measurement is usually performed on an unknown sample by measuring
the reflectance of the sample and performing, for example, a
Levenberg-Marquardt optimization, as shown in "Numerical Recipes in
C (2.sup.nd Edition)", (W. H. Press, S. A. Teukolsky, W. T.
Vetterling, and B. P. Flanery, Cambridge University Press,
Cambridge 1992), with the film thickness and EMA fraction treated
as optimization parameters.
[0037] A production reflectometer typically does not directly
measure the incident intensity (as provided from the source or the
actual incident intensity on the measured sample), which is
required to measure reflectance of an unknown sample, but instead
will determine the incident intensity from the reflected intensity
of a known calibration sample. The incident intensity can change
over time due to variations in source intensity, environment
(temperature and humidity), drift in optical alignment, and the
like. A known calibration sample, often a silicon wafer with its
native oxide, is first measured, and its reflectance assumed to be
known. The incident intensity is determined by dividing the
intensity reflected from the calibration sample by its assumed
reflectance. The reflectance for an unknown sample is then
determined by measuring the intensity reflected from the sample and
dividing by the incident intensity.
[0038] Obviously, such a calibration method depends on the
stability of the calibration sample. In VUV regions, stability is
not guaranteed, since small differences in native oxide thicknesses
are magnified in that region. In addition, the previously mentioned
contamination that occurs confounds the stability of the
calibration sample, since the photodeposition occurs every time the
calibration sample is measured.
[0039] One way to deal with this problem has already been discussed
with reference to the calibration techniques disclosed in the U.S.
patent applications disclosed above. A measurement of reflectance
for a thick (.about.1000 .ANG.) silicon dioxide on silicon
substrate sample relative to a thin oxide sample (typically native
silicon dioxide on silicon substrate) is independent of incident
intensity, and can be used along with a regression technique to
determine both the native oxide thickness as well as contaminant
thickness on the thin oxide sample. The result of this analysis is
used to calculate the reflectance of the native oxide calibration
sample, R.sub.c, which is used in combination with the intensity
reflected from the calibration sample, I.sub.c, to determine the
incident intensity via I.sub.0=I.sub.c/R.sub.c. The reflectance of
an unknown sample, R.sub.s, can then be determined from its
reflected intensity, I.sub.s, by R.sub.s=I.sub.s/I.sub.0.
[0040] Disclosed herein is an alternate method for measuring thin
film properties that uses reflectance ratios to bypass the system
calibration completely. As used herein, the term "calibration"
refers to the determination of incident intensity, I.sub.0. The
method disclosed herein can lead to better long-term performance
for some thin film systems, one example being thickness and
concentration in ultra-thin silicon oxynitride.
[0041] One embodiment of the technique involves measuring the
reflected intensity of three samples:
[0042] Sample 1--native oxide/Si reference piece,
[0043] Sample 2--.about.1000 .ANG. SiO.sub.2/Si reference
piece,
[0044] Sample 3--the unknown sample (for example an oxynitride
sample).
[0045] The unknown sample will normally consist of a standard
silicon substrate of 150 mm, 200 mm, 300 mm, or 450 mm diameter
with a deposited film stack. As shown in FIG. 3, the VUV
reflectometer discussed in the prior art (U.S. Pat. Nos. 7,026,626,
7,067,818, 7,126,131, and 7,271,394, which are expressly
incorporated herein by reference in their entirety) is equipped
with a stage and loading port for accepting and measuring reflected
intensity at various locations on such a sample which may be placed
in sample area 302. The two reference pieces may be small pads,
such as pad 1 303 and pad 2 304, mounted on the stage, or at some
other location convenient for the wafer/chuck system 301. This
reduces the measurement of the reference pieces to basically moving
to their locations and collecting intensity data, with no
additional wafer handling. FIG. 3 shows an illustration of this
arrangement 300. As shown in FIG. 3, the reference pieces are
provided integrally with the stage or wafer/chuck system 301 or
other sample holder or the like. It will be recognized that the
concepts described herein may be utilized with any reference
samples and such reference samples do not have to be integrally
provided with the stage or wafer/chuck system 301. Thus, as
described herein reference pieces such as pads 303 and 304 may be
referred to, however, it will be recognized that any reference
sample may be provided having the characteristics of the reference
pieces.
[0046] The unknown sample is loaded into the system 301, and
reflected intensities, I1, I2, and I3, are measured for Sample 1,
Sample 2, and Sample 3 (for example Sample 1 being pad 1 303 and
Sample 2 being Pad 2 304), respectively. Two ratios are formed:
I2/I1=R2/R1, and
I2/I3=R2/R3. Eq. 1
[0047] The equalities are true as long as I.sub.0 has not changed
significantly during the measurement of the reflected intensities.
I.sub.0 is usually stable for at least several minutes, meaning
that several locations on Sample 3 could be measured and use the
same I1 and I2 in the ratios. I1 and I2 need only be measured with
whatever frequency a standard system calibration would normally be
performed. An additional embodiment might incorporate the current
method and the calibration methods disclosed in U.S. patent
application Ser. Nos. 10/930,339, 11/418,827, 11/418,846, and
11/789,686 simultaneously, which are expressly incorporated herein
by reference in their entirety. The same pads 303, 304 can be used
for calibration of I.sub.0 or used as described in the current
disclosure, depending on the particular measurement being done.
Other ratio combinations can obviously be used as well. As
described in more detail herein, the techniques provided herein are
particularly advantageous in that the reference pieces need not be
stable under the conditions that the reflectometer operates. Thus,
reference pieces that, for example, are not stable in the VUV
regime may still be utilized. For example, even though the
contaminate build-up which may affect a VUV measurement may occur
on the reference piece, rendering the reference piece unstable in
VUV conditions, the reference piece is still suitable for the
techniques described herein.
[0048] Thus, during operation instability of the reference sample
may relate to the surface of the reference sample changing over
time, such as for example, but not limited to contaminant buildup,
airborne molecular contaminant removal, growth of films, other time
dependent changes, etc. In addition, instability of the reference
sample may also relate to inherent non-uniformities of the
reference sample (across a given sample or from sample to sample),
that may result, for example, from the sample production
techniques. For example, bare thicknesses, native oxides, interface
properties, surface roughness conditions, etc. may all initially
vary across a sample and from sample to sample. Thus these may not
change over time, however, from sample to sample or across a sample
these conditions may be considered unstable. Thus, as used herein,
instability may refer to both time dependent and non-time dependent
variations.
[0049] The reason for framing the problem in terms of reflectance
ratios instead of intensity ratios is that reflectance can be
calculated in a straight-forward manner using standard thin film
algorithms, as described in "Spectroscopic Ellipsometry and
Reflectometry-A User's Guide", (H. Tompkins and W. McGahan, John
Wiley & Sons Press, New York, published 1999), along with
values for the optical properties and thicknesses of the various
films. For instance, if the SiO.sub.2 and Si optical properties are
known and SiO.sub.2 thicknesses provided, the reflectances R1 and
R2 can be calculated. Going further, if a measured R2/R1 is
available, standard regression techniques can be used to optimize
the thicknesses for the SiO.sub.2 layers, giving a measurement for
both thicknesses, as long as the parameters are sufficiently
decoupled. In principle, the optical properties of the SiO.sub.2
and Si layers could be determined as well, normally using
parameterized dispersion models such as the Tauc-Lorentz model, as
shown in "Parameterization of the optical functions of amorphous
materials in the interband region", (G. E. Jellison and F. A.
Modine, Appl. Phys. Lett., Vol. 69 (1996), p. 371).
[0050] In one embodiment, the techniques disclosed herein may be
utilized in combination with the techniques disclosed in U.S.
patent application Ser. Nos. 10/930,339, 11/418,827, 11/418,846,
and 11/789,686. For example, a measurement software routine may be
selectable between differing modes, a first mode being the
techniques described herein and a second mode being the techniques
described in the above mentioned U.S. patent applications. Thus,
the system may selectably operate (automatically or based on user
input) in at least one of a plurality of measurement modes, the
plurality of measurement modes including at least a first
measurement mode and a second measurement mode. The first
measurement mode may be configured to utilize a combination of the
unknown sample and reference sample data that is independent of
incident intensity to determine a property of the unknown sample,
without calibrating incident reflectometer intensity as described
herein in more detail. The second measurement mode may be
configured to utilize the reference sample data in a manner that is
independent of incident intensity to determine one or more
properties of one or more reference pieces, thereby determining the
incident intensity of the reflectometer, after which reflectance of
unknown samples may be determined such as described in the above
referenced U.S. patent application Ser. Nos. 10/930,339,
11/418,827, 11/418,846, and 11/789,686.
[0051] The current method disclosed herein involves a regression
analysis of both ratios in Equation 1 simultaneously. Basically,
the parameters in the modeled ratios are optimized until both
calculated ratios R2/R1 and R2/R3 agree with their corresponding
measured ratios. One way to do the optimization is to use a version
of the Levenberg-Marquardt routine generalized to multiple sample
analysis. In such cases, the nonlinear chi-square merit function
could be written as:
.chi. 2 = i = 1 N 21 ( 1 .sigma. i ) 2 ( ( R 2 R 1 ) i , measured -
( R 2 R 1 ) i , calculated ) 2 + j = 1 N 23 ( 1 .sigma. j ) 2 ( ( R
2 R 3 ) j , measured - ( R 2 R 3 ) j , calculated ) 2 Eq . 2
##EQU00001##
[0052] where the .sigma..sub.i and .sigma..sub.j are estimates of
the standard error for each measured data point. The notation on
the summation limits, N21 and N23, illustrates that the data range
for the two datasets does not have to be the same.
[0053] The results of the optimization procedure are the measured
parameters for all three samples. The reference pads 303, 304 will
ordinarily undergo contaminant buildup due to extended use in the
system, and so a contaminant layer will be included in the
reflectance models for the reference pieces. Thus the result of the
analysis include the thicknesses of both oxide (native and
.about.1000 .ANG.) thicknesses, thickness of contaminant in both
reference pieces, and all of the same regression parameters for the
unknown sample that would have been varied during a standard
optical measurement, such as film thicknesses and optical
properties (via the EMA fraction in the ultra-thin SiON case). The
redundancy provided by having sample 2 involved in both datasets
helps constrain the problem and yield better results for the
unknown sample.
[0054] A series of simulations will follow to illustrate the
usefulness of the method in the case of ultra-thin silicon
oxynitride (SiON) gate films, which serve the role of Sample 3. For
the purposes of this description, the optical properties n and k of
the silicon native oxide, silicon dioxide (SiO.sub.2), silicon
(Si), Silicon Nitride (Si.sub.3N.sub.4), and contaminant are
regarded as known. The optical values were taken from a variety of
literature sources or determined through other measurements. In
particular, the contaminant optical properties could be determined
using a controlled experiment similar to the methods disclosed in
U.S. patent application Ser. No. 11/789,686, which is expressly
incorporated herein by reference in its entirety. The SiON films
are treated as Bruggeman EMA films composed of SiO.sub.2 and
Si.sub.3N.sub.4. Aside from the optical properties, a full
description of the ultra-thin oxynitride film is considered to be a
specification of its thickness and EMA volume fraction. The volume
fraction can be correlated to nitrogen content in the films, which
is an important process control parameter along with the film
thickness. In the present example, treatment of explicit interface
layers and surface and interface roughness are ignored, but such
effects could also be included in the models, if desired.
[0055] FIGS. 4A and 4B show simulations of the variation of the
ratio R2/R1, where R2 is the simulated reflectance of a 1000 .ANG.
SiO.sub.2 on Si substrate and R1 is the simulated reflectance of 10
.ANG. SiO.sub.2 (plot 401), 20 .ANG. SiO.sub.2 (plot 402), and 30
.ANG. SiO.sub.2 (plot 403) on Si substrate samples. FIG. 4A shows a
relative reflectance range of 0 to 1.4, and a wavelength range of
120 nm to 1000 nm. FIG. 4B is an expanded version of a portion of
FIG. 4A, and shows a relative reflectance range of 0 to 1.4, and a
wavelength range of 120 nm to 400 nm. FIGS. 5A and 5B show similar
ratios with R1 fixed at 20 .ANG. SiO.sub.2 on Si substrate and R2
varied from 1000 .ANG. SiO.sub.2 (plot 501), 1010 .ANG. SiO.sub.2
(plot 502), and 1020 .ANG. SiO.sub.2 (plot 503) on Si substrate.
FIG. 5A shows a relative reflectance range of 0 to 1.4, and a
wavelength range of 120 nm to 1000 nm. FIG. 5B is an expanded
version of a portion of FIG. 5A, and shows a relative reflectance
range of 0 to 1.4, and a wavelength range of 120 nm to 400 nm.
Clearly, the effects of changing thickness of the thin and thick
oxides in these ratios are decoupled, and may be readily extracted
from measured ratios through regression procedures. The measured
ratio is simply the ratio of the reflected intensities of the two
samples, which is independent of I.sub.0 if only a short time has
passed between the intensity measurements.
[0056] FIGS. 6A, 6B, 7A, 7B, 8A, and 8B show the effects of
contaminant buildup on both calibration samples. In FIGS. 6A and
6B, a reflectance ratio of 10 .ANG. contaminant on 1000 .ANG. SiO2
on Si and 10 .ANG. contaminant on 10 .ANG. SiO2 on Si is shown as
plot 601, 20 .ANG. contaminant on 1000 .ANG. SiO2 on Si and 20
.ANG. contaminant on 10 .ANG. SiO2 on Si is shown as plot 602, and
30 .ANG. contaminant on 1000 .ANG. SiO2 on Si and 30 .ANG.
contaminant on 10 .ANG. SiO2 on Si is shown as plot 603. FIG. 6A
shows a relative reflectance range of 0 to 1.4, and a wavelength
range of 120 nm to 1000 nm. FIG. 6B is an expanded view of FIG. 6A,
and shows a relative reflectance range of 0 to 1.4, and a
wavelength range of 120 nm to 400 nm. The optical properties for
the contaminant layer were determined from a prior reflectance
ratio analysis study. FIGS. 7A and 7B illustrate that the effect of
increasing contaminant buildup on the native oxide sample is to
primarily increase the ratio in the VUV, as the reflectance of the
native oxide sample decreases. In FIGS. 7A and 7B, a reflectance
ratio of 10 .ANG. contaminant on 1000 .ANG. SiO2 on S and 10 .ANG.
contaminant on 10 .ANG. SiO2 on Si is shown as plot 701, 10 .ANG.
contaminant on 1000 .ANG. SiO2 on Si and 20 .ANG. contaminant on 10
.ANG. on SiO2 on Si is shown as plot 702, and 10 .ANG. contaminant
on 1000 .ANG. SiO2 on Si and 30 .ANG. contaminant on 10 .ANG. SiO2
on Si is shown as plot 703. FIG. 7A shows a relative reflectance
range of 0 to 1.4, and a wavelength range of 120 nm to 1000 nm.
FIG. 7B is an expanded version of a portion of FIG. 7A, and shows a
relative reflectance range of 0 to 1.4, and a wavelength range of
120 nm to 400 nm. In contrast, the effect of growing contaminant on
the 1000 .ANG. SiO.sub.2/Si sample, as seen in FIGS. 8A and 8B, is
to increase the interference amplitude minima and simultaneously
shift the locations of the interference minima to longer
wavelengths. In FIGS. 8A and 8B, a reflectance ratio of 10 .ANG.
contaminant on 1000 .ANG. SiO2 on Si and 10 .ANG. contaminant on 10
.ANG. SiO2 on Si is shown as plot 801, 20 .ANG. contaminant on 1000
.ANG. SiO2 on Si and 10 .ANG. contaminant on 10 .ANG. SiO2 on Si is
shown as plot 802, and 30 .ANG. contaminant on 1000 .ANG. SiO2 on
Si and 10 .ANG. contaminant on 10 .ANG. SiO2 on Si is shown as plot
803. FIG. 8A shows a relative reflectance range of 0 to 1.4, and a
wavelength range of 120 nm to 1000 nm. FIG. 5B is an expanded
version of a portion of FIG. 8A, and shows a relative reflectance
range of 0 to 1.4, and a wavelength range of 120 nm to 400 nm.
[0057] Comparisons of FIGS. 6A, 6B, 7A, 7B, 8A, and 8B with FIGS.
4A, 4B, 5A, and 5B also show that the contaminant buildup is
decoupled from changes in the 1000 .ANG. SiO.sub.2/Si thickness.
The contaminant is also decoupled from the thin oxide thickness,
although the effects on the ratio are more subtle. In practice, the
regression procedure is able to extract the correct changes, and
this method is effective at accounting for changes in reflectance
of the 1000 .ANG. and native oxide calibration samples without
knowing the changes a priori.
[0058] FIGS. 9A-12B show simulations of several R2/R3 ratios. FIGS.
9A and 9B show 1000 .ANG. SiO.sub.2 (plot 901), 1010 .ANG.
SiO.sub.2 (plot 902) and 1020 .ANG. SiO.sub.2 (plot 903) on silicon
(R2) relative to a 30 .ANG., 15% EMA volume fraction SiON on
silicon film as R3, illustrating the effects of changing SiO.sub.2
thickness on the R2/R3 ratio. FIG. 9A shows a relative reflectance
range of 0 to 1.4, and a wavelength range of 120 nm to 1000 nm.
FIG. 9B is an expanded version of a portion of FIG. 9A, and shows a
relative reflectance range of 0 to 1.4, and a wavelength range of
120 nm to 400 nm.
[0059] FIGS. 10A and 10B show ratios with 0 .ANG. contaminant
buildup (plot 1001), 10 .ANG. contaminant buildup (plot 1002), and
20 .ANG. of a contaminant buildup (plot 1003) on the 1000 .ANG.
SiO.sub.2 on silicon sample, with R3 the same 30 .ANG., 15%
fraction SiON film as in FIGS. 9A and 9B. FIG. 10A shows a relative
reflectance range of 0 to 1.4, and a wavelength range of 120nm to
1000 nm. FIG. 10B is an expanded version of a portion of FIG. 10A,
and shows a relative reflectance range of 0 to 1.4, and a
wavelength range of 120 nm to 400 nm.
[0060] FIGS. 11A and 11B show the effects of changing SiON
thickness (29 .ANG. (plot 1101), 30 .ANG. (plot 1102), 31 .ANG.
(plot 1103), 15% EMA fraction) on the R2/R3 ratio, and FIGS. 12A
and 12B show the effects of changing EMA % (30 .ANG., 13% (plot
1201), 15% (plot 1202), 17% (plot 1203) EMA fractions) on the
ratio. FIG. 11A shows a relative reflectance range of 0 to 1.4, and
a wavelength range of 120 nm to 1000 nm. FIG. 11B is an expanded
version of a portion of FIG. 11A, and shows a relative reflectance
range of 0 to 1.4, and a wavelength range of 120 nm to 220 nm. FIG.
12A shows a relative reflectance range of 0 to 1.4, and a
wavelength range of 120 nm to 1000 nm. FIG. 12B is an expanded
version of a portion of FIG. 12A, and shows a relative reflectance
range of 0 to 1.4, and a wavelength range of 120 nm to 220 nm.
[0061] If Sample 2 did not change, the reflectance of Sample 3
could be extracted directly from the ratios in FIGS. 11A, 11B, 12A,
and 12B. If one inspects the figures closely, it is apparent that
the effect of changing SiON thickness is to decrease the VUV
portion of the R3 spectrum (with corresponding increase in R2/R3),
and the effect of changing the EMA % is to bend the shape of the R3
spectrum, with anchor points near 120 nm and 220 nm. However,
Sample 2 is not stable, but builds up contaminant over time, the
effect of which was shown in FIGS. 10A and 10B. An analysis of
R2/R3 alone might reasonably be expected to exhibit some coupling,
especially between contaminant thickness on the Sample 2 piece and
EMA % of the SiON film. Analyzing the R2/R1 ratio simultaneously
with the R2/R3 ratio helps to constrain the possible values of R2
contaminant thickness, since the properties of R2 are the same for
both ratios. This in turn enhances the determination of the R3
properties.
[0062] An example of a simultaneous multiple ratio fit of a SiON
film is shown in FIGS. 13A-13D. The raw data consists of reflected
intensities from two reference pieces consisting of native oxide
and .about.1000 .ANG. SiO.sub.2 films on silicon, and a central
location on a SiON sample. As described above, the ratios R2/R1
measured (plot 1301) and modeled (plot 1302) shown in FIGS. 13A and
13B, and R2/R3 measured (plot 1304) and modeled (plot 1305) shown
in FIGS. 13C and 13D were simultaneously analyzed, resulting in
optimized parameters for all three samples. The results of the
optimization shown in FIGS. 13A-13D are 12.041 .ANG. contaminant
and 19.242 .ANG. SiO.sub.2 for Sample 1, 7.275 .ANG. contaminant
and 1045.8 .ANG. SiO.sub.2 for Sample 2, and 31.709 .ANG. thickness
and 16.036% nitrogen for Sample 3. The fit parameters for R2 were
constrained to be the same for both ratios. FIG. 13A shows a
relative reflectance range of 0 to 1.5, and a wavelength range of
120 nm to 600 nm. FIG. 13B is an expanded version of a portion of
FIG. 13A, and shows a relative reflectance range of 0 to 1.5, and a
wavelength range of 120 nm to 220 nm. FIG. 13C shows a relative
reflectance range of 0 to 2, and a wavelength range of 120 nm to
600 nm. FIG. 13D is an expanded version of a portion of FIG. 13C,
and shows a relative reflectance range of 0 to 2, and a wavelength
range of 120 nm to 220 nm.
[0063] In some embodiments, the underlying oxide and possibly even
interface regions of the reference pieces can be pre-characterized
using a ratio measurement or other means, and those parameters
fixed to the pre-characterized values during normal measurements.
After such pre-characterization, only the contaminant layer on the
reference pieces and properties of the unknown sample would be
treated as unknowns in multiple ratio measurements. A further
generalization might treat multiple contaminant layers, due to
different types of photodeposited contaminants, or to distinguish
the effects of photocontaminants from airborne molecular
contaminants, which are known to absorb on wafer surfaces in normal
fab environments.
[0064] An experiment demonstrating the effectiveness of the
disclosed method consisted of 5 SiON samples, each measured at 5
measurement sites/wafer per day for 10 days. The measurement sites
were slightly changed locally on the SiON samples each day to
prevent photocontaminant buildup on the SiON samples themselves
from affecting the results. The results for standard deviation of
the 10 day measurements for each site are a metric of the stability
for the SiON measurement. Photocontamination was allowed to occur
on the two reference pieces. These conditions simulate the way the
SiON process would be monitored in a fab production
environment--i.e. each SiON sample would only be measured once,
while the reference pieces would likely be used for many
measurements, and consequently undergo the photocontamination
process.
[0065] Each of the 250 measurements consists of 3 reflected
intensities--one each from the two reference pads and one from the
SiON measurement site. The data was first analyzed by calibrating
I.sub.0 using a dual pad calibration procedure with the two
reference pads (similar to methods discussed in patent application
Ser. Nos. 11/418,827, 11/418,846, and 11/789,686), and the
thickness and percent nitrogen (via the EMA fraction) were analyzed
using an EMA model and standard reflectance analysis. The 10-day
standard deviation was computed for thickness and percent nitrogen
for each site of each sample. The data was then recomputed using
the multiple ratio analysis method described in this disclosure.
The same optical models were used for reference and SiON materials
for the recomputed data. The current method resulted in an average
improvement in the 10-day standard deviation of approximately 37%
for thickness and 26% for nitrogen percent.
[0066] In practice, similar stability enhancements can also be
achieved through further optimization of the contaminant
properties, or even alternate choices in calibration materials. The
significance of this study lies in the fact that a stability
enhancement was achieved using the disclosed method with the same
reference pads, without further optimization of the reference or
SiON material descriptions.
[0067] It is noted that the SiON description used for the analysis,
in particular the oxide and nitride component optical properties,
was generated using standard reflectance measurements by
calibrating I.sub.0. The good fit in FIGS. 13C and 13D are an
indication that the previous analysis was largely successful.
However, multiple SiON samples, each using a multiple ratio
analysis, could be used to further refine the optical description
of the SiON film, and consequently improve the fits in FIGS. 13C
and 13D. In this case, the oxide and nitride component optical
properties of the SiON film would be included as fit parameters,
along with thickness and EMA fraction. The use of multiple SiON
samples with different thicknesses helps to constrain the
determination of the oxide and nitride component optical
properties. This would likely result in even further improvement of
stability results for both multiple ratio and calibrated
reflectance measurements.
[0068] As previously mentioned, one particularly attractive feature
of the current method is that it may be combined with a multiple
pad calibration procedure using the same or even additional
reference pads on a single measurement platform. The multiple ratio
method used may depend on the particular film measurement being
done. In other words, whether or not to calibrate I.sub.0 and
generate reflectance or to use a multiple ratio calculation
instead, or even which multiple ratio method to use, could be
recipe dependent. FIG. 14 shows a generalized version of FIG. 3,
where multiple reference pads, such as pad 1 1403, pad 2 1404, pad
3 1405, pad 4 1406, and pad 5 1407, each with different film
characteristics are available for use depending on the sample being
measured. The wafer/chuck system 1401 comprises a sample area 1402
similar to as described above. The intensities from any number of
the reference pads could be used along with the sample intensity in
any combination that does not depend on I.sub.0 (not only limited
to intensity ratios) and allows for accurate extraction of the
desired sample parameters.
[0069] It is noted that the current method has been illustrated
using a specific example, and one will recognize that many
variations on the current procedure are possible, while still
remaining within the scope of this disclosure. Additionally, the
method described herein has been described for use with VUV
reflectometer measurements, for which it is particularly
advantageous, but the concept is valid for reflectance measurements
carried out at any wavelength. The method described herein has also
described a moving stage and sample holder, and can obviously be
conceived to include automation via robotic wafer handling, fab
interface software, and any number of other common modifications of
optical metrology equipment for manufacturing environments.
[0070] Further modifications and alternative embodiments of the
techniques disclosed herein will be apparent to those skilled in
the art in view of this description. It will be recognized,
therefore, that the techniques disclosed herein are not limited by
these example arrangements. Accordingly, this description is to be
construed as illustrative only and is for the purpose of teaching
those skilled in the art the manner of carrying out the techniques
disclosed herein. It is to be understood that the forms of the
techniques disclosed herein shown and described are to be taken as
the presently preferred embodiments. Various changes may be made in
the implementations and architectures. For example, equivalent
elements may be substituted for those illustrated and described
herein, and certain features of the techniques disclosed herein may
be utilized independently of the use of other features, all as
would be apparent to one skilled in the art after having the
benefit of this description of the techniques disclosed herein.
* * * * *